Keep bone rotated in-between two other bones?
While the method you mentioned works, it can get messy especially if you want more influences on one bone. Here is a method which is simple and easy to read.

An advantage is, that the hierarchy and bone orientation doesn't matter since it uses World Space constraints.


Duplicate the between bone and rename it with suffixes of the other bones.

Add Copy Rotation constraints from both duplicated joints onto the between joint.
Set the influence of the second copy rotation constraint to 0.5. The between bone is now equally influenced by the other two.
Parent the between_end bone to the end bone.Parent the between_start bone to the start bone.The between bone is now influenced by the original bone. But we're still using World Space constraints but don't have to worry about the orientations.


The rig holds various poses. Parenting can be changed freely as long as nothing is parented to the between bone (cyclic dependency).



The rig will break, if the angle of the original bones is more than 180°.





I recommend using World Space constraints with duplicates, because you are free to change the orientation later on. With local space constraints the bones have to be oriented the same, which is not always given.